The term gas exchange characterizes the periodic change of the cylinder charge, that is, the discharge of exhaust gas and the inflow into the combustion chamber of air or air/fuel mixture. In conventional internal combustion engines, the control of the gas exchange takes place via resiliently-biased valves which are opened by a camshaft. The opening function is a time-dependent function, that is, the start, duration and extent of the opening are determined by the form of the camshaft. The opening function can therefore be precisely fixed by the shape of the camshaft but is not variable.
Additional systems are known for controlling the exchange of gas with a variable opening function. International patent publication WO 91/08384 corresponds to U.S. Pat. No. 4,957,074 and discloses an electromagnetically controlled and hydraulically actuated valve having a variable opening function. German patent publication 195 01 386 corresponds to U.S. Pat. No. 5,588,411 and discloses a variable valve control wherein the opening function of the gas-exchange valve is determined by an opening camshaft and a closing camshaft. The opening function can be varied within wide limits via a change of the phase shift between the shafts rotating at the same rotational frequency.
The operability of the variable valve control or the operability of the change of the opening function is relevant for the emission of the exhaust gas and must therefore be monitored.